Shared code that was tested on Android and iOS.
Using Xamarin Forms 188.8.131.5290 with a NavigationPage.
Steps to Reproduce:
1.) Set a page as your MainPage for Navigation.
2.) Observe OnAppearing() is called on Android and iOS
3.) Push another page to Navigation with PushModalAsync()
4.) Pop the newly pushed page using PopModalAsync()
5.) Observe OnAppearing() is NOT called on Android for your original MainPage but is on iOS.
If you Push the 2nd page normally instead of modal Android works correctly.